Heretofore, a laminated composite film obtained by laminating various plastic films with a metal foil using an adhesive has been used widely in the field of packaging. Particularly, a laminated composite film comprising a metal foil is excellent in light shielding properties of the metal foil and gas and liquid barrier properties and is therefore used widely as a packaging material for foods to be subjected to a sterilization treatment at high temperature.
As the adhesive for a laminate used to laminate the laminated composite film, a two-component curable urethane adhesive comprising a base resin containing a polyesterpolyol and a curing agent containing a polyisocyanate is widely used in view of handiness and excellent adhesion performance.
The laminated composite film comprising a metal foil is usually produced in the form of a three-layered structure by laminating a drawn plastic film (PET, NY, etc.) on one surface of a metal foil using an adhesive for a laminate and laminating an undrawn plastic film (CPP, etc.) on the other surface of the metal foil using an adhesive for a laminate. Contents such as food are usually packed so as to contact with the side of the undrawn plastic film. There also may be a case of using a four-layered structure laminated composite film produced by further laminating another drawn plastic film on the inner or outer side so as to contact with the metal foil.
Use of a solvent has been restricted recently so as to improve working environment. Therefore an organic solvent-based adhesive has been replaced by a solventless adhesive in production of the laminated composite film. As the solventless adhesive, a two-component curable solventless adhesive containing a polyol as a base resin and a polyisocyanate as a curing agent is used.
However, a molecular weight of the component in the solventless adhesive is far smaller than that of the component in the solvent-based adhesive. Therefore, the solventless adhesive may cause a problem that when it is used in the production of the laminated composite film comprising a metal foil such as an aluminum foil which requires a sterilization treatment at high temperature, an adhesive strength of the side of the metal foil, which contacts with foods, drastically decreases after the sterilization treatment at high temperature.
Particularly, when foods containing an acid component such as vinegar is packed, as contents, such a phenomenon appears more remarkably. Furthermore, when the packaging material is stored for a long period, there may arise problems in presenting pear skin appearance and peeling of the plastic film from the metal foil of the laminated composite film.
Under these circumstances, the two-component curable solventless adhesive has hitherto been used restrictly in the field of dry foods and snack foods which do not require a heat treatment, and it is partly used as a packaging material comprising no metal foil in applications which require a sterilization treatment at high temperature.
Therefore, in the two-component curable solventless adhesive, a method of acid-modifying hydroxyl groups of a polyol with an acid anhydride has been proposed as means for improving adhesion between the metal foil such as an aluminum foil and a plastic film.
In Japanese Examined Patent Publication (Kokoku) No. 7-94654, for example, there is proposed an adhesive composition comprising a polyol or a polyurethanepolyol and a polyurethane polyisocyanate, which comprises a polyol having a carboxyl group obtained by reacting a polyol having three or more functional hydroxyl groups with a polycarboxylic acid so as to remain at least two hydroxyl groups, as the polyol, and a polyurethanepolyol obtained by copolymerizing a carboxyl group-containing polyol, which is obtained by reacting a polyol having three or more functional hydroxyl groups with a polycarboxylic acid so as to remain at least two hydroxyl groups, as the polyurethanepolyol.
In Japanese Patent No. 3,584,480, for example, there is proposed a solventless adhesive composition for a film-film composite laminate comprising an organic polymerpolyol compound selected from polyesterpolyol, polyetherpolyol, polyetheresterpolyol and polyurethanepolyol each having a number average molecular weight of 300 to 5000, an organic polymerpolyol compound having a carboxyl group introduced by adding a carboxylic anhydride to the organic polymerpolyol compound, and a polyisocyanate compound, wherein a content of the carboxylic anhydride is from 0.3 to 1 parts by weight based on 100 parts of a solid content of the organic polymerpolyol compound.
However, when hydroxyl groups of the polyol are acid-modified or an anhydride of carboxylic acid is added in the two-component curable solventless adhesive, workability usually deteriorates due to a rapid increase in viscosity. A decrease in the amount of hydroxyl groups to be acid-modified causes a problem such as pear skin, and thus peeling of the metal foil from the plastic film may occur.